1. Field of the Invention
This invention relates to an electronic balance equipped with a windshield case, and more particularly to an electronic balance having a high-resolution for eliminating a difference in a buoyancy between a weight for calibration incorporated in a balance body and a measuring object placed on a weighing pan within a weighing chamber.
2. Description of the Related Art
In an electronic balance having high accuracy permitting a very fine minimum readout value, high resolution and automatic sensitivity calibrating function, influence of air convection and temperature change due to the wind from outside may cause an error. In such an electronic balance, in order to remove the cause of an error, a method of encircling the periphery of a weighing pan by a windshield case is adopted. This electronic balance equipped with the windshield case, as shown in a schematic configuration diagram of FIG. 8, generally comprises a weighing chamber 93 accommodating a weighing pan 91 and encircled by a windshield case 92. The electronic balance further comprises a balance body 94 incorporating a calibration weight which is a weight for calibration, its adding/removing mechanism, a load detecting section and the like. The weighing chamber 93 and the balance body 94 are separated by a floor 95 of the weighing chamber 93. The weighing pan 91 and the load detecting section are coupled by means of an on-pan load transmitting shaft (not shown).
Then, after a predetermined temperature change or a predetermined time passage, the calibration weight is automatically applied to the load detecting section. On the basis of the output produced at this time and a previously stored mass of weight, the sensitivity coefficient is corrected and updated to a new sensitivity coefficient. (For example, refer to JP-B-6-52190.)
The electronic balance of the related art is constructed as described above. Meanwhile, the balance body 94, which is provided with an incorporated component and a case, has a larger heat capacity than the weighing chamber 93. Therefore, when an ambient temperature or the temperature of the balance body 94 drastically changes, temperature difference is likely to occur between the weighing chamber 93 and the balance body 94. For example, when the electronic balance is actuated in a state where the indoor air-conditioning is stopped in which the electronic balance is placed, since the weighing chamber 93 has a smaller heat capacity, its internal temperature can relatively easily catch up with the change of the air-conditioning temperature. On the other hand, since the balance body 94 has a larger heat capacity, the balance body 94 has a poor track-ability for the air-conditioning temperature. Thus, a temperature difference occurs between the weighing chamber 93 and the balance body 94. Specifically, in summer, since the air-conditioning temperature is lower than an outdoor temperature, the temperature in the weighing chamber 93 is likely to be lower than that of the balance body 94. Inversely, in winter, since the air-conditioning temperature is higher than the outdoor temperature, the temperature in the weighing chamber 93 is likely to be higher than that of the balance body 94.
As described above, if the temperatures within the weighing chamber 93 and the balance body 94 are different, the respective air densities within the weighing chamber 93 and balance body 94 are also different. Thus, a difference occurs between the buoyancy which a measuring object placed on the weighing pan 91 receives and that which the calibration weight receives, which results in an error in the measured value.